The present invention relates to a spectroscope designed to obtain a spectral distribution of the incident light by Fourier-transforming a spatial interferogram that is formed by a quadrangular common path interferometer. More particularly, the present invention relates to a Fourier transform spectroscope with a quadrangular common path interferometer which is capable of highly sensitive spectroscopic detection of light from a luminous source with a finite area and, hence, is suitable for spectroscopic detection of extremely weak luminescence, for example, bioluminescence, chemiluminescence, fluorescence from a biological specimen, etc.
Conventional spectroscopes may be roughly classified into the following three types: a dispersive spectroscope that employs a spectroscopic prism or a diffraction grating; a temporal Fourier transform spectroscope designed to obtain a spectral distribution of the incident light by Fourier-transforming the temporal signal of a temporal interferogram that is formed by moving a moving mirror of a Michelson interferometer; and a spatial Fourier transform spectroscope designed to obtain a spectral distribution of the incident light by Fourier-transforming a spatial interferogram that is formed by a double beam interferometer, for example, a quadrangular common path interferometer.
Referring to FIG. 13, in the conventional spatial Fourier transform spectroscope that employs a quadrangular common path interferometer,light from a light source 1 that is placed at the front focal point of a condenser lens L1 with a focal length f is converted into a parallel light beam through the condenser lens L1 and then divided into two beams, that is, transmitted light and reflected light by a beam splitter BS. The transmitted light returns to the beam splitter BS via mirrors M3, M2 and M1 and passes it and is then focused by an imaging lens L2 with a focal length f to form a light source image once at the back focal point of the lens L2. Thereafter,the light, which is now in the form of a divergent light beam, enters a one-or two-dimensional photodetector D that is disposed at a position conjugate with the mirror M2. Meanwhile, the reflected light from the beam splitter BS returns it via the mirrors M1, M2 and M3 in the opposite direction to the above. The light is reflected therefrom and then focused by the imaging lens L2 to form a light source image once at the back focal point of the lens L2. Thereafter, the light enters the photodetector D where it interferes with the above-described transmitted light, thereby forming a spatial interferogram on a detecting surface of the photodetector D. The distance between the successive interference fringes of the interferogram is determined by the inclination angle .theta. of the mirror M2 from the 45.degree. plane. The interferogram signal that is obtained by the one- or two-dimensional photodetector D is subjected to spatial Fourier transform to analyze the spatial frequency distribution of the signal, thereby obtaining a spectral distribution of the light source 1.
Incidentally, the spatial Fourier transform spectroscope is superior in comparison with the temporal Fourier transform spectroscope, as described below. Therefore, it may be considered practical to use the conventional Fourier transform spectroscope employing a quadrangular common path interferometer such as that shown in FIG. 13. However, it is not necessarily possible to say that the conventional quadrangular common path interferometer has a satisfactory optical arrangement. In particular, it needs two lenses, that is, the condenser lens L1 and the imaging lens L2, and it is necessary to dispose the two-dimensional photodetector D at a position conjugate with the mirror M2.
FIG. 14 shows an equivalent optical path diagram of the conventional Fourier transform spectroscope with a quadrangular common path interferometer. Assuming that a plane that passes through the center of the mirror M2 and perpendicularly intersects the optical axis is defined as a plane A, the detecting surface of the photodetector D as a plane B, the distance between the plane A and the imaging lens L2 as a', and the distance between the imaging lens L2 and the plane B as b', the planes A and B have a positional relationship to each other in terms of the image formation, which is given by EQU 1/a'+1/b'=1/f
However, the light source 1 and the plane A are not in imagery positional relation to each other. Noting the fact that light beams which travel counter to each other are inclined at 4.theta. relative to each other by the mirror M2, the interference occurring on the plane B may be considered equivalent to that divergent light beams which are emitted from the respective light sources 1 disposed on respective axes intersecting each other at the position of the plane A with an inclination 4.theta. relative to each other are converted into parallel light beams through the respective condenser lenses L1, and these two plane waves enter the common imaging lens L2 with an inclination 4.theta. relative to each other and are once condensed to the back focal point of the lens L2, and then the light beams diverging therefrom interfere with each other to form Young's interference fringes on the plane B (interference plane) that is conjugate with the plane A.
However, as will be clear from the arrangement shown in FIG. 14, the conventional Fourier transform spectroscope with a quadrangular common path interferometer enables interference fringes of high contrast (contrast ratio of about 1) to be obtained on the plane B (also on another plane) only when the light source 1 can be approximated to a point source. If the light source 1 has a finite area, since the plane A is not a geometrical optics image of the light source 1, light at each point on the plane A is superposition of light rays from various points of the light source 1. Accordingly, since the plane B is conjugated with the plane A, light rays from the entire area of the light source 1 are superimposed at each point on the plane B in the same way as in the case of the plane A. As the light source 1 increases in size, the number of light rays superimposed increases. Therefore, although the plane B is a plane where interference fringes can be formed, the contrast lowers as the light source 1 increases in size.